Optical Magnifiers (also referred to as “eyepieces” or “loupes”) are known. Typically, these optical devices are used to allow direct viewing of slides or other small objects or are used as part of other optical systems such as, for example, telescopes and viewfinders.
Conventional optical magnifiers utilize magnifier lenses that provide an enlarged virtual image of a real object in front of a viewer's eye. It is generally preferable that such lenses, in combination with the object being viewed, provide an apparent field of view to the user in excess of +/−10 degrees, in order to avoid the sensation of “tunnel vision”. Additionally, these magnifier lenses preferably provide a relatively long eye relief (that is, the distance at which the lens can be held from the eye) in order to allow an object to be comfortably viewed.
Optical magnifiers have also been suggested for viewing electronic displays incorporated, for example, in portable electronic devices. When used to view electronic displays, the same criteria, described above, applies, even though newer high quality micro-displays are now being manufactured with full diagonals of 6 mm or less. In order to obtain a +/−10 degree apparent field of view, such small micro-displays require a high magnification lens (on the order of 15× or greater), which translates to an effective focal length of approximately 17 mm or less. As is known, magnification for this type of optical system is calculated using the standard formula: M=254 mm/EFL, where EFL is the effective focal length of the lens, measured in mm.
For comfortable viewing by users, including those wearing eyeglasses, it is generally accepted that a reasonable eye relief is approximately 17 mm or greater. As such, in the relative sense, the eye relief should preferably be at least as great as the EFL of the lens (for example, 17 mm in the 15× example described above) for micro-displays of this size. This relationship between eye relief and EFL becomes even more of a concern when shorter focal length (higher magnification) systems are contemplated. This is a new and challenging goal that did not previously exist for larger electronic displays used with correspondingly lower magnification lenses.
In U.S. Pat. No. 4,094,585, E. I. Betensky discloses a three-element all-plastic optical magnifier comprising from the viewing end, a first positive lens group comprising a single element, and a second lens group comprising a bi-convex element and a bi-concave element forming a doublet having the overall shape of a meniscus. This magnifier has a magnification in the range of 13× to 14×. For micro-displays with full diagonals of less than 6 mm, this magnifier does not provide the desired +/−10 degree field of view. Additionally, this magnifier has the added labor expense of cementing two elements to form a doublet, which may be unacceptable for cost-sensitive applications.
In U.S. Pat. No. 5,835,279, I. Marshall and R. Holmes disclose a three-element all-plastic magnifier lens for viewing an LCD in the binocular vision system of a head-mounded display unit. This design has a large (+/−35.8 degree) apparent field of view and a long (17 mm) eye relief. However, the LCD is quite large (33.65 mm full diagonal) and the resulting magnification is only about 11×, making it unsuitable for use with micro-displays of the scale contemplated here. Additionally, when this system is scaled to a magnification of 15×, the eye relief drops to about 12.4 mm, quite short for users wearing eyeglasses.
In U.S. Pat. Nos. 5,909,322 and 5,886,825, J. R. Bietry discloses two- and three-element plastic designs for magnifier lenses suitable for use in liquid crystal (LCD) or light emitting diode (LED) micro-display systems. Although these lenses have a magnification of 16×, high image quality, and long eye relief, these designs contain at least one diffractive surface, for both the two element and three element forms and/or include a rear meniscus lens which is concave toward the object and positionable within 5 mm of the object surface. As a number of newer “micro-display” devices, for example, LCD devices, depend on light being delivered from the front of the display via a polarizing beam splitting device, a back focal length (BFL) of the lens in excess of 5 mm is often essential for the placement of the optical magnifier. For micro-displays which require a BFL in excess than 5 mm, these designs will simply not function.